1. Field of the Invention
The present invention relates to shaped polyamide articles, such as fibers, films, sheets or the like, containing dyes, pigments or modifying additives, and processes for manufacturing the same which are excellent in melt-molding performance.
2. Prior Art
As a process for mass-coloring thermoplastic fiber-forming polymers represented by polyethylene terephthalate and nylon-6, a process wherein pigments or dyes are admixed during the polymerization reaction and a color concentrate process wherein pigments or dyes in a high concentration are dispersed in the polymer have heretofore been well-known. However, as for the former, the colorants in practical use are limited to pigments having a good heat resistance, such as titanium dioxide, carbon black or the like, because of high temperatures in the reaction system. In addition, it has drawbacks such that installation of an exclusive autoclave is required because of staining of the autoclave and that lots of expense and labor are required for cleaning.
As for the latter, in the manufacture of color concentrate, drying of polymers before processing and drying of the color concentrate after processing are required, so that energy consumption is large. Particularly in the case of polyamides, there are disadvantages that properties of melt-spun fibers, such as tensile strength, elongation or the like, deteriorate or that knots (polymer agglomerates) are formed along the filaments, owing to the thermal history experienced in kneading and drying processes.
Alternatively, a dry coloring process wherein pigments or dyes are kneaded directly with a polymer is also known. However, it has difficulties caused by powdery pigments or dyes such that drying is hardly performed, pigments or dyes fly and dispersibility is poor. In any case, cleaning of spinning lines and drying equipment is required in the case of color changing, so that this process is difficult to adapt to frequent color changings.
The inventors paid their attention to the use of a liquid coloring mixture as a measure for obviating the above drawbacks of the coloring processes. A liquid coloring mixture is usually prepared by kneading a pigment or dye with a liquid vehicle at normal temperature and a constant quantity of this mixture is fed by a gear pump or plunger pump and compounded with a polymer in a molten state. This liquid coloring mixture has been applied to polyolefins such as polyethylene, polypropylene or the like, ABS resins and polyesters (for example, as described in Japanese Patent Application Laid-open Nos. 60-45,689 and 61-9,453). However, for polyamides, it has been found that only Japanese Patent Application Laid-open No. 59-75,926 and U.S. Pat. No. 4,167,503 disclose the use of polyoxyethylene alkyl phenylethers as a vehicle and Japanese Patent Application Laid-open No. 63-92,717 discloses polyamide fibers mass-colored with a liquid colorant comprising an isoindolinone pigment, a metal stearate and a vehicle (preferably, a polyester-ether). As a result of an investigation of the above conventional techniques, the inventors have found that polyamide fibers or films manufactured according to those conventional techniques still present problems, such as migration of or staining by dyes and pigments, bleeding of vehicles, opacification of fibers or films, filament breakage due to poor dispersibility or compatibility of dyes and pigments, or the like. Thus, in the present situation, excellent liquid coloring mixtures for polyamides have not yet been developed and, therefore, mass-colored polyamide fibers, sheets or films are scarcely in commercial production.